Switches for power distribution, in particular for low voltages, are generally known as circuit breakers, compact switches, etc., and, in their function as protective switches, are used to automatically interrupt the currents flowing through the switch. The switch is generally designed for the three phases of a three-phase power supply system, where the current in the neutral phase (the neutral conductor) frequently also flows through the switch. During normal operation, the currents in each phase each flow via switching contacts which rest on one another and can be opened at the same time by way of a switching shaft in order to interrupt the currents.
The known switches furthermore have an electronic release (for example an overcurrent release), which ensures that the switching contacts open in predetermined (tripping) conditions (tripping situation: for example overcurrent). The electrical power for the release's own power supply is taken by way of electrical power converters (generally iron-core transformers) from the phase currents to be monitored, to be precise via the magnetic fields, based on the transformer principle. Current sensors in the form of instrument transformers are used to record the individual phase currents, that is to say for the actual current recording/measurement, and generally use an air-cored coil (Rogowski converter), converting the magnetic field of the associated phase conductor to a measurement signal which is proportional to the current differentiated with respect to time.
In order to assess the measurement signals, the release has an electronic circuit which is generally in the form of a microprocessor. The purpose of the release is to identify the occurrence of a tripping situation and to interrupt all the current flows by way of the switching shaft. Nowadays, so-called combination instrument transformers are generally used in switches, that is to say a power converter and an instrument transformer are provided for each phase to be monitored.
The known switches have the disadvantage that the instrument transformers may be defective, and a tripping situation is therefore not identified. This may be caused by wire fractures in the supply lines between the instrument transformer and the release, and by turns shorts in the coil windings. Turns shorts in particular do not at the same time lead to a total failure, but to incorrect measurement signals. In this case, it is relatively complex to test the switches during operation.